Verify that mount didn't fail, fix CVE-2017-7496
    
    Since /tmp is world writable, and we do not verify the return code
    of mount (or mkdir), a local attacker could precreate /tmp/boot
    and /tmp/root directories in a way that mount fail, and later
    add symlink to fool the script.  For example /tmp/root/etc/ pointing to
    /etc , which in turn will make this sed call
    
      sed -i 's/root:x:/root::/' /tmp/root/etc/passwd
    
    modify the real /etc/password file (since the tool is running
    as root, SELinux policies are most likely bypasswed, and the symlink
    protection in /tmp wouldn't be applicable if the symlink is directory).
    
    Thanks to Adam Mariš from Red Hat Product Security for helping me
    verify the attack vector.